Semiconductor device and fabrication method thereof

ABSTRACT

A semiconductor device and a fabrication method thereof are provided. The semiconductor device includes a semiconductor substrate which comprise a first type well and a second type well, and a plurality of junction regions therebetween, wherein each of the junction regions adjoins the first and the second type wells. A gate electrode disposed on the semiconductor substrate and overlies at least two of the junction regions. A source and a drain are in the semiconductor substrate oppositely adjacent to the gate electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims priority of Taiwan Patent Application No.97110055, filed on Mar. 21, 2008, the entirety of which is incorporatedby reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating asemiconductor device, and in particular relates to a method forfabricating a semiconductor device for increasing operation voltage.

2. Description of the Related Art

High voltage MOS transistors are widely used in many electronic devices,such as central processing unit voltage supply devices, power supplymanager system devices and AC/DC inverters and the like. Because highvoltage MOS transistors are usually operated under high operationvoltage, a high electric field may be formed, resulting in a largenumber of hot electrons near the junction region of the channel and thedrain. The hot electrons will excite the electrons near the drain to theconduction band to form an electron-hole pair, thereby affectingcovalent electrons near the drain. Most of the electrons ionized by hotelectrons may move to the drain to increase drain current (I_(sub)), anda small portion of the ionized electrons may be injected into andtrapped by the gate oxide, resulting in changing the threshold voltageof a gate electrode. Additionally, the holes resulting from hotelectrons may flow to the substrate to produce a drain current(I_(sub)). Thus, when the operation voltage increase, the number ofelectron-hole pair increases and results in “carrier multiplication”.

FIG. 1 shows a cross-section view of a traditional high voltage MOStransistor with a lateral diffused drain. As FIG. 1 shows, a highvoltage MOS transistor 130 is formed on a semiconductor wafer 110. Thesemiconductor wafer 110 has a P-type silicon substrate 111 and a P-typeepitaxial layer 112 formed on the P-type silicon substrate 111. The highvoltage MOS transistor 130 has a P-type well 121, N-type source region122 formed in the P-type well 121, an N-type drain region 124 formed inthe P-type epitaxial layer 112, and a gate electrode 114.

When the drain current mentioned above flows through the P-type siliconsubstrate 111, the resistance (R_(sub)) of the P-type silicon substrate111 may produce an induced voltage (V_(b)). If the induced voltage islarge enough, forward bias may occur between the P-type siliconsubstrate 111 and the source region 122 to form a parasitic bipolartransistor 140. When the parasitic bipolar transistor 140 is turned on,the current from the drain region 124 flowing to source region 122 israpidly increased, resulting in electrical breakdown to cause the highvoltage MOS transistor 130 to malfunction.

In some high voltage MOS devices, in order to provide a high voltage,“double diffused drain” structures are used in the source and drain.FIG. 2 shows a high voltage MOS transistor with a double diffused drainstructure disclosed by U.S. Pat. No. 5,770,880. A substrate 210 has anN-type body 212. A gate 220 on a gate oxide 222 is formed between asource 230 and a drain 240. The source and drain are substantially thesame and interchangable, therefore only the drain is described in theflowing. Every drain has a double diffuse region comprising a firstheavily doped contact region 214 and a lightly doped region 216. Thediffusion regions are formed by implanting P-type ions such as boroninto the exposed surface of the substrate after forming an open 219 onthe oxide layer and performing an annealing process to make P-type ionsdiffuse into the substrate 210 to form the doped regions 214 and 216.The contact region 214 is usually limited on the surface of the N-typebody 212 and do not extend into the N-type body 212. The second lightlydoped region 216 extends into the N-type body 212 and a portion of thesecond lightly doped region 216 is under the gate electrode 220. Ajunction region is formed between the doped region 216 and N-type body212 and the junction region determines the breakdown voltage value ofthe device. The diffusion doped region 216, having a low dopingconcentration gradient, may decrease the reverse bias electric fieldnear the body-drain junction region. Specifically, this allows thedevice to operate under a high voltage before reaching the breakdownvoltage. However, fabricating the device mentioned above requires acomplicated process and additional masks may be needed, thus increasingcosts. Therefore, a new semiconductor device and a fabrication methodthereof are needed to improve the breakdown voltage of the devicewithout incurring extra costs.

BRIEF SUMMARY OF THE INVENTION

The invention provides a method for fabricating a semiconductor device,comprising: providing a semiconductor substrate; forming a first typewell in the semiconductor substrate; and forming a second type well anda plurality of junction regions in the semiconductor substrate, whereineach of the junction region is between the first and the second typewells, and adjoins the first and the second type wells.

The invention also provides a semiconductor device, comprising: asemiconductor substrate comprising a first type well and a second typewell, and a plurality of junction regions therebetween, wherein each ofthe junction regions adjoins the first and the second type wells; a gateelectrode on the semiconductor substrate and overlies at least two ofthe junction regions; and a source and a drain in the semiconductorsubstrate are oppositely adjacent to the gate electrode.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more fully understood by reading thesubsequent detailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 is a cross section view of a traditional semiconductor device;

FIG. 2 is a cross section view of a traditional semiconductor device;

FIGS. 3-9 are cross section views illustrating the step for fabricatinga semiconductor device according to an embodiment of the invention;

FIG. 10A shows a drain voltage-drain current measurement value of atraditional semiconductor device; and

FIG. 10B shows a drain voltage-drain current measurement value of anexample of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

Reference will be made in detail to the present embodiments, examples ofwhich are illustrated in the accompanying drawings. Wherever possible,the same reference numbers are used in the drawings and the descriptionto refer to the same or like parts. In the drawings, the shape andthickness of one embodiment may be exaggerated for clarity andconvenience. This description will be directed in particular to elementsforming part of, or cooperating more directly with, apparatus inaccordance with the present invention. It is to be understood thatelements not specifically shown or described may take various forms wellknown to those skilled in the art. Further, when a layer is referred toas being on another layer or “on” a substrate, it may be directly on theother layer or on the substrate, or intervening layers may also bepresent.

FIGS. 3-9 are cross section views illustrating the step for fabricatinga semiconductor device according to an embodiment of the invention.

Referring to FIG. 3, first, a semiconductor substrate such as a P-typesubstrate 100 is provided. The P-type substrate 100 is preferably asilicon substrate. In other embodiments, the P-type substrate 100comprises SiGe, silicon on insulator (SOI) substrate or othersemiconductor material substrates. Then, a lithography process isperformed and a photoresist layer 15 a is applied on the P-typesubstrate 100. After that, a mask 500, comprising an opaque area 60 anda transparent area 61, is provided. Light 5 is then made to pass throughmask 500 to perform an exposure process to transfer a pattern on themask 500 onto the photoresist layer 15 a on the P-type substrate 100.

As FIG. 4 shows, a development is performed and a portion of thephotoresist layer 15 a which is not covered by opaque area 60 is removedto form a patterned photoresist layer 15 b. The patterned photoresistlayer 15 b is used to define a predetermined area of the first type ionimplant region 16.

FIG. 5 illustrates the P-type substrate 100, wherein a first type ionimplant is performed by using the patterned photoresist layer 15 b as amask to form a first type well 102 in the P-type substrate 100. Thefirst type ions mentioned above may be N-type or P type ions.

In the embodiment, the steps of forming a mask 500 comprise firstproviding a first integrated circuit layout database comprising data ofthe first type well and then forming the mask 500 by using the firstintegrated circuit layout database.

Referring to FIG. 6A, after removing the patterned photoresist layer 15b, a photoresist layer 18 is blanketly deposited on the P-type substrate100. After that, a mask 600 comprising an opaque area 70 and atransparent area 71 is provided. Light 6 is then made to pass throughmask 600 to perform an exposure process to transfer a pattern on themask 600 onto the photoresist layer 18 on the P-type substrate 100.

As FIG. 7 shows, a development is performed and a portion of thephotoresist layer 18 which is not covered by the opaque area 70 isremoved to form a patterned photoresist layer 19. The patternedphotoresist layer 19 is used to define predetermined areas of the secondtype ion implant region 21 and junction region 22. A second type ionimplant is performed on the P-type substrate 100 by using the patternedphotoresist layer 19 as a mask to form a second type well 104 and aplurality of junction regions 106 a in the P-type substrate 100. Thesecond type ions mentioned above may be N-type or P type ions and havean opposite conductive type to the first type ions. It is noted that,the mask 600 and the mask 500 have patterns which are complementary toeach other. Therefore, by adjusting ranges of opaque areas andtransparent areas of the two masks, the plurality of junction regions106 a may be formed between the first type well 102 and the second typewell 104 and each of the junction regions adjoins the first and thesecond type wells. The steps of forming the mask 600 comprise providinga second integrated circuit layout database comprising data of the firsttype well 102, data of the second type well 104 and data of theplurality of junction regions 106 a, then accessing the secondintegrated circuit layout database and performing a Boolean logicoperation to obtain an operation result, and finally using the operationresult to form the mask 600. The length of the plurality of junctionregions is about 0.2-5 μm, preferably 0.5-1.5 μm.

Referring to FIGS. 3-7 again, in the embodiment, an additional opaquearea (Fig. not shown) is formed between opaque areas 60 and 70 byreducing the opaque range of the opaque areas 60 (as FIG. 3 shown) ofthe mask 500 or the opaque areas 70 of the mask 600 (as FIG. 6A shown).This may laterally extend the range of the first type well 102 and thesecond type well 104, resulting in edges of the first type well 102 andthe second type well 104 having doped overlapped regions. Therefore,after completing the first ion implant 20 and the second ion implant 30,respectively, a plurality of junction regions 106 a, which are bothdoped with the first and second type ions, may be formed. In oneembodiment, the implant dosage of the first type ion implant 20 isgreater than that of the second type ion implant 30, and thus a lightlydoped first type ion region may be formed in the junction regions 106.In other embodiment, a lightly doped second type ion region may beformed in the junction regions by doping second type ions having aconcentration higher than that of the first type ions.

FIG. 5 and FIG. 6B illustrate another embodiment of forming junctionregions in the P-type substrate 100. Compared with the embodiment inFIG. 7, the opaque range of the opaque area 80 of the mask 700 is largerthan that of the opaque area 70 of the mask 600, and thus the range ofthe first type well 102 and the second type well 104 may be laterallyreduced to form a plurality of junction regions 106 b without doping thefirst and second type ions mentioned above, after completing the firstion implant 20 and the second ion implant 40. In other words, thejunction regions have substantially the same conductive type with theP-type substrate 100.

FIG. 7 and FIG. 8 illustrate a plurality of isolation structures, suchas a shallow trench isolation (STI) structure 74, formed in the P-typesubstrate 100 to define a device region 200. In general, the steps offorming the shallow trench isolations comprise forming a trench, fillingthe trench with a dielectric material such as a high-density plasmaoxide, and then performing a planarization process such as a chemicalmechanical polishing process to remove the excess dielectric material toform the shallow trench isolations. However, the isolation structuresmay also be field oxides (FOX) formed by local oxidation of silicon.

FIG. 9 illustrates a MOS device 116 formed on device region 200. The MOSdevice 116 further comprises a gate dielectric layer 125. In onepreferred embodiment, the gate dielectric layer 125 comprises an oxidelayer, and the gate dielectric layer 125 may be formed by a process suchas a dry or wet thermal oxidation oxide process in the atmosphere withoxide, water, NO or the combinations thereof, or by a CVD process usingtetraethoxysilane (TESO) and oxygen as a precursor. The steps of formingthe MOS device 116 comprise first forming a gate electrode 120 on theP-type substrate 100, wherein the gate electrode 120 overlies at leasttwo of the junction regions 106 a, then forming a source 123 and a drain124 in the semiconductor substrate which is oppositely adjacent to thegate electrode. The source 123 and drain 124 may be formed by well-knownion implant processes and the source 123 and drain 124 have the sameconductive type with the first type well 102.

The gate electrode 120 preferably comprises the conductive material ofTa, Ti, Mo, W, Pt, Al, Hf, Ru, or silicide or nitride thereof. In onepreferred embodiment, the gate electrode 120 is composed of polysiliconand may be formed by depositing doped or undoped polysilicon through aCVD process.

The gate electrode 120 and gate dielectric layer 125 may be patterned,for example, by a lithography process. In general, the lithographyprocess comprises applying a photoresist material, then masking,exposing, and developing the photoresist material to form a photoresistmask. After patterning the photoresist mask, an etch process isperformed to remove the unwanted portion, thus forming the gateelectrode 120 and gate dielectric layer 125 mentioned above.

Similarly, in other embodiments, using the method mentioned above, a MOSdevice 116 may be formed with a gate electrode 120, a gate dielectriclayer 125, a source 123, and a drain 124 on the P-type substrate 100 ofthe embodiment in FIG. 6B, wherein the gate electrode 120 overlies atleast two of the junction regions 106 b (not shown).

It is noted that because the junction regions 106 a and 106 b arebetween the first type well 102 and the second type well 104, the PNjunctions may be formed between the second type well 104 under thesource 123 and the second type well 104 under the gate electrode 120,and between the second type well 104 under the drain 124 and the secondtype well 104 under the gate electrode 120, respectively. A depletionregion may be formed in the second type well 104 under the source 123and/or drain 124 and gate electrode 120 by the PN junctions. With thedepletion region, breakdown voltage may increase during operation andthe range of operation voltage of the device may be increased.

Referring to FIGS. 10A and 10B, under different gate operation voltage,drain voltage-drain current measurement values of a traditionalsemiconductor device and an embodiment of semiconductor device of theinvention, respectively, are shown. As FIG. 10A shows, the gateoperation voltage (V_(g)) is about 0-45 V. However, as shown in FIG.10B, the gate operation voltage (V_(g)) of an embodiment ofsemiconductor device of the invention may be raised to about 0-60 V.Specifically, compared with the conventional semiconductor device, therange of the gate operation voltage may be raised to 30% by using theembodiment of semiconductor device of the invention. Moreover, anadditional process is not needed in the method for fabricating theembodiment of semiconductor device. Processes substantially the samewith well-known processes may be used, and thus, costs are notincreased.

While the invention has been described by way of example and in terms ofthe preferred embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements (aswould be apparent to those skilled in the art). Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

What is claimed is:
 1. A method for fabricating a semiconductor device,comprising: providing a semiconductor substrate; forming a first typewell in the semiconductor substrate; and forming a second type well anda plurality of junction regions in the semiconductor substrate, whereineach of the junction regions is between the first and the second typewells, and adjoins the first and the second type wells, and wherein thestep of forming the first type well, the second type well and theplurality of junction regions, comprises: forming a first photoresistlayer on the semiconductor substrate; providing a first mask; using thefirst mask to perform an exposure to transfer a pattern on the firstmask onto the first photoresist layer on the semiconductor substrate;using the first photoresist layer as a mask to perform a first type ionimplant to form the first type well in the semiconductor substrate;removing the first photoresist layer; forming a second photoresist layeron the semiconductor substrate; providing a second mask; using thesecond mask to perform an exposure to transfer a pattern on the secondmask onto the second photoresist layer on the semiconductor substrate;and using the second photoresist layer as a mask to perform a secondtype ion implant to form the second type well and the plurality ofjunction regions in the semiconductor substrate, wherein each of thejunction region is between the first and the second type wells, andadjoins the first and the second type wells.
 2. The method as claimed inclaim 1, further comprising: forming a gate electrode on thesemiconductor substrate; and forming a source and a drain in thesemiconductor substrate which is oppositely adjacent to the gateelectrode.
 3. The method as claimed in claim 2, wherein the gateelectrode overlies at least two of the junction regions.
 4. The methodas claimed in claim 1, wherein the step of forming the first maskcomprises: providing a first integrated circuit layout databasecomprising data of the first type well; and forming the first mask byusing the first integrated circuit layout database.
 5. The method asclaimed in claim 1, wherein a method for forming the second maskcomprises: providing a second integrated circuit layout databasecomprising data of the first type well, data of the second type well anddata of the plurality of junction regions; accessing the secondintegrated circuit layout database and performing a Boolean logicoperation to obtain an operation result; and using the operation resultto form the second mask.
 6. The method as claimed in claim 1, whereinthe first type well is a P-type well or an N-type well.
 7. The method asclaimed in claim 1, wherein the second type well is a P-type well or anN-type well and has an opposite conductive type to the first type well.8. The method as claimed in claim 1, wherein the length of the pluralityof junction regions is about 0.2-5 μm.
 9. The method as claimed in claim1, wherein the plurality of junction regions are doped with both P-typeions and N-type ions, and the plurality of junction regions is formed bythe first type ion implant which forms the first type well and by thesecond type ion implant which forms the second type well.
 10. The methodas claimed in claim 9, wherein implant dosage of the P-type ions isgreater than that of the N-type ions.
 11. The method as claimed in claim9, wherein the implant dosage of the P-type ions is less than that ofthe N-type ions.
 12. The method as claimed in claim 1, wherein theplurality of junction regions has substantially the same conductive typewith the semiconductor substrate, and the plurality of junction regionsis formed by the first type ion implant which forms the first type welland by the second type ion implant which forms the second type well. 13.The method as claimed in claim 1, wherein the semiconductor substratefurther comprises a plurality of isolation structures.
 14. The method asclaimed in claim 13, wherein the isolation structures are shallow trenchisolations formed by a shallow trench process.
 15. The method as claimedin claim 13, wherein the isolation structures are field oxides formed bylocal oxidation of silicon.